The present invention relates to a device for eliminating current surge through switch contacts and the like when the contacts are initially closed. More particularly, the invention relates to a negative temperature-resistance characteristic device with a first order resistivity transformation for use in delaying current flow through current carrying switch contacts when they are initially closed.
A common problem in switch contacts, particularly leaf-spring mounted contacts such as relay switch contacts, is that they deteriorate over a period of time due to arcing. This arcing is due to a number of causes such as bouncing of the contacts when they first close. In the case of bouncing, the switch contacts usually do not completely mate at the first touch, but instead the movable contact tends to bounce several times before it reaches a permanent, stable state with respect to the fixed contact. Such bouncing increases the transfer of conductive material between the switch contacts and the formation of a series of electric arcs. This is especially true when the contacts are in series with an incandescent lamp load, since the initial current flow can be up to ten times larger than the steady state flow.
In some prior art devices the arcing effects of bouncing are eliminated by elaborate and expensive designing of the relay contacts and their supporting frames. In still other prior art devices the arcing effects of bouncing are eliminated by connecting a relatively high resistance in series with the switch contacts. The high resistance prevents a significant amount of current from flowing through the relay contacts when they are first closed. After the contacts have ceased their bouncing, the resistance is shunted out of the circuit by the closing of a separate switch. This type of device has several disadvantages, such as requiring some extra means for closing the shunt switch (which may also be subject to arcing) and increasing the complexity of the relay switching circuit.
Recently a temperature-sensitive polyconductor compound has been developed which demonstrates a negative temperature-resistance characteristic. In this type of compound, a relatively high electrical resistance is presented, provided the compound has a temperature below a predetermined critical temperature which is unique for each material. When the temperature of the compound reaches this critical temperature, a first order transformation occurs with an atomic rearrangement within the unit cell which often results in an increase in crystal symmetry of the material. In a strain free homogeneous crystal this transformation can be observed to take place over a negligible temperature interval, that is in much less than a degree centigrade. As a result of this transformation the electrical resistance is sharply reduced by several orders of magnitude, for example to 1/1000 of its high resistance value, and it essentially thereafter becomes conductive. The material again resumes its high resistance state when the temperature of the compound is reduced below the critical temperature.
Polyconductive compounds of this type are described further in U.S. Pat. Nos. 3,402,131 to Futaki and 3,532,641 to Chamberland. Some prior uses of this type of polyconductor have been in temperature type alarm systems in which the compound is connected in series with an alarm so that when the ambient temperature surrounding the compound reaches the critical temperature, such as in the case of a fire, the alarm is activated.
The present invention overcomes the above and other disadvantages of prior art relay contact protector devices by a novel design utilizing a negative temperature-resistance polyconductor which additionally simplifies construction of the device.